U.S. Pat. No. 4,314,031 (Sanford et al.) discloses a family of tin-phosphorus oxyfluoride glasses. These glasses are low temperature glasses that characteristically melt at temperatures on the order of 400.degree.-450.degree. C. and exhibit very low Tg values. Generally, these values are under 200.degree. C. and frequently below 100.degree. C. In weight percent on an elemental basis, the Sanford et al. glasses are composed essentially of 20-85% Sn, 2-20% P, 3-20% O and 10-36% F, the total of the four elements being at least 75%. A number of optional constituents are disclosed, the principal one being up to 25% Pb.
The low Tg values of these glasses suggested their use for low temperature sealing purposes. However, the high coefficients of thermal expansion (CTE) exhibited by these glasses, generally on the order of 200.times.10.sup.-7 /.degree.C., severely limited the areas of potential use. The solving of this problem is described in some detail in pending application Ser. No. 07/598,001, filed Oct. 9, 1990 by Cornelius et al. now U.S. Pat. No. 5,089,446, and assigned to the assignee of the subject application. The subject matter of that application is incorporated herein by reference.
As described in the Cornelius et al. application, the high CTE problem was attacked by both a glass composition study and a mill addition study. The glass composition study revealed that the incorporation of up to 11% niobium (Nb) in the base glass composition had the unique effect of substantially lowering the CTE value of the glass. Furthermore, this occurred without appreciable change in the Tg value. Further study showed that the unique effect of Nb could be further enhanced by additions of Mo, Fe, or V to the niobium glass.
At the same time, the mill addition approach revealed several additions that were compatible, that is, they remained inert in the glass, and decreased the effective CTE. These additives included Invar, molybdenum, tungsten, lead orthophosphate and a pyrophosphate containing one or more cations selected from Mg, Co, As, Zn, Al, Fe and Zr.
The pyrophosphates are of particular interest for at least two reasons. First, they can be obtained by crystallizing a glass of appropriate composition. Second, their effectiveness derives from large negative CTEs due to steep inversion temperatures. Each of the effective pyrophosphates has a different inversion temperature. This permits considerable flexibility in selecting, or combining, inversion points over the range of 50.degree. to 350.degree. C.
Attempts to put into practice the sealing materials of the Cornelius et al. application encountered problems. Differences in densities of the components led to separation and settling on standing. Further, the conventional binders of the art, nitrocellulose and methylcellulose, failed to burn off properly at low sealing temperatures.
It is then a basic purpose of our invention to provide solutions to these problems. Another purpose is to provide an improved sealing mixture that can be cast as a sealing tape, and the resulting improved sealing tape.